A redox flow battery (RFB) stores electrical energy in reduced and oxidized species dissolved in two separate electrolyte solutions, the anolyte and the catholyte. The anolyte and the catholyte circulate through a cell electrode separated by a membrane or separator. Redox flow batteries are advantageous for energy storage because they are capable of tolerating fluctuating power supplies, repetitive charge/discharge cycles at maximum rates, overcharging, overdischarging, and/or because cycling can be initiated at any state of charge.
However, among the many redox couples upon which redox flow batteries are based, a number of disadvantages exist. For example, many systems utilize redox species that are unstable, are highly oxidative, are difficult to reduce or oxidize, precipitate out of solution, and/or generate volatile gases. One of the main challenges confronting RFB systems is the intrinsically low energy density compared with other reversible energy storage systems, such as lithium-ion batteries. Additionally, many redox flow battery systems use electrolytes containing transition metals, such as the vanadium RFBs, which increases the cost of the electrolyte and thus the overall system. Therefore a need exists for RFB systems having a greater energy density and/or a lower cost electrolyte.